Mixer-settler extractors have problems including generally higher construction cost and larger holdup in the system than tower extractors. For extractions in a limited number of stages, however, they call for less investments for equipment than the tower type. Especially with systems which involve low extraction rates or wide oil-water viscosity difference, or in cases where ingress of impurities can take place, they exhibit quite excellent characteristic performance.
In the extraction of a solute from its aqueous solution by the use of an organic extractant, such as a mixture of a tertiary amine and an organic diluent, the solvent viscosity is so high and the rate of solute travel is so low that it is not customary to employ a tower extractor, typically a perforated-plate or packed tower, which is devoid of any mechanically working part. For such an application the tower extractor must be tall enough to accommodate the very high actual height of each theoretical plate. Moreover, in order to prevent troubles such as flooding, the operation range is limited and a large-diameter extraction tower must be used, thereby causing economic disadvantage.
Thus, if efficient extraction is to be done using an organic solvent, especially an extractant system containing a tertiary amine, it is advisable to employ an extractor of, e.g. a mixer-settler type, which is designed to accelerate the frequencies of dispersion and coalescence of liquid droplets.
For the extracting operation of a mixer-settler extractor, attainment of the highest possible rate of mixed liquids separation in the settler is of prime importance. The higher the separation rate the smaller the settler size may be. The smaller settler will accordingly require a narrower installation area and may hold less liquid than ordinary settlers. These are substantial economic advantages where the organic solvent for use as an extractant and/or the construction material for the settler is expensive.